1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead that includes a lead body with a shape-memory polymeric annular seal and a shape-memory polymeric suture sleeve.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. Most such leads include an elongated flexible tubular, electrically insulating sleeve that is connected proximally to a connector that is adapted to couple to the header of the cardiac stimulator can, and distally to a tip electrode that is positioned near the tissue requiring stimulation. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices.
At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed. The implantation route and site are usually imaged in real time by fluoroscopy to confirm proper manipulation and placement of the lead.
Prior to closure, a suture sleeve (if not already in place) is slipped over the lead sleeve, positioned proximal to the site of transvenous entry, and sutured in place to body tissue. The suture sleeve is designed to anchor a portion of the lead to a preselected portion of tissue proximal to the site of transvenous entry and to restrict the longitudinal movement of the lead following implantation. The design goal is not to eliminate all longitudinal movement of the lead since some longitudinal movement thereof is inevitable due to normal chest and heart movements and to physical exertion. Rather, the aim is to prevent radical longitudinal movements of the lead that could dislodge the tip electrode from the endocardium or even fracture the conductor wires inside the lead. The movement restricting function of conventional suture sleeves is accomplished by tying one or more ligature sutures around the suture sleeve that clamp the suture sleeve to the exterior of the lead sleeve.
One difficulty associated with conventional suture sleeve design is the potential for stress risers created by the ligature sutures that clamp the suture sleeve to the lead sleeve. The crimping action of a ligature suture is spread over a very small area. Thus, the ligature point can act as a fulcrum for bending movement of the wires inside the lead sleeve. Fatigue failure of the wires can result. If the suture is tied too tightly, metal fatigue may be accelerated or worse, the wires may be damaged at the time of implantation. While a skilled physician can often avoid overtightening, there remains the residual problem of a large crimping force applied to a very small area.
In addition to irregular stresses imparted on the lead, conventional suture sleeves are susceptible to becoming unclamped from the lead sleeve. The problems stems from the gradual loosening of the ligature sutures over time. A suture may loosen due to a variety of causes, such as improper suturing by the physician, stretching of the suture material, heavy exertion by the patient or other causes. Regardless of the particular origin, an unclamped suture sleeve can no longer prevent the implanted lead from making large longitudinal movements. Such unchecked movements may result in the tip electrode detaching from the endocardium.
As noted above, the lead must be connected to the cardiac stimulator prior to closure. This entails inserting the connector into a passage in the header of the cardiac stimulator and manipulating some type of anchoring mechanism, such as a set screw, to hold the connector in place. Most conventional connectors include one or more external O-rings to provide a seal against the penetration of body fluids into the passage after implantation. Like many types of O-rings used as fluid seals in other industrial applications, conventional connector O-rings are molded with a circular or elliptical profile and an outer diameter based on an anticipated inner diameter of the header passage. The outer diameter of the O-ring is chosen to be slightly larger than the inner diameter of the passage. When the connector is pushed into the passage, the O-rings elastically deform to conform to the passage. Some effort is normally required to insert the connector even where the dimensions of the O-rings and the passage are closely matched.
Difficulties arise with conventional connector O-rings where the dimensions of the O-rings, the header passage, or both fall outside manufacturing tolerances. Where the O-rings are too large in relation to the passage, insertion may be difficult and require force that may ultimately damage the O-rings. Where the O-rings are too small in relation to the passage, an inadequate seal may be formed. Because of the rather small dimensions involved, even a small imperfection in the seal of the O-ring is often amplified due to capillary action.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.